Due to high spectrum efficiency, strong anti-multipath interference capability and low cost, the OFDM technique gains more and more attentions and is recommended as the preferred technique of the broadband wireless communication. In the present OFDM systems, the data transmission method is very important. For a typical OFDM system, a data transmission unit (such as one frame of OFDMA system based on 802.16, or a subframe in LTE OFDM system) has several OFDM symbols, and at the front of each OFDM symbol, there is a cyclic prefix which is usually the repeat of last data segment of the OFDM symbol. With the cyclic prefix, the OFDM system can not only eliminate the inter-symbol interference caused by multiple paths, but also ensure that the carriers are orthogonal with each other to guarantee the spectrum utility efficiency of the OFDM system. Therefore, the setting of the cyclic prefix in the data transmission unit is very important and how to reasonably set the cyclic prefix has profound significance for the OFDM system.
In the current OFDM system, there are two data transmission methods often used to set the cyclic prefix:
One method is: “3GPP, TR25.81, Physical Layer Aspects for Evolved UTRA (release 7)” provides a setting of cyclic prefix in the LTE OFDM system after the W-CDMA system. When a subframe has 7 OFDM symbols, the settings of cyclic prefixes with different system bandwidths are shown in the following table:
TABLE 1the setting of cyclic prefix in 3GPP, TR25.814Bandwidth1.25 MHz2.5 MHz5 MHz10 MHz15 MHz20 MHzThe setting(4.69/9) ×(4.69/18) ×(4.69/36) ×(4.75/73) ×(4.73/109) ×(4.75/146) ×of cyclic6,5,3,6,2,5,prefix(5.21/10) × 1(4.95/19) × 2(4.82/37) × 4(4.82/74) × 1(4.77/110) × 5(4.79/147) × 2
Taking the setting of cyclic prefix with the bandwidth of 5 MHz in Table 1 as an example, “(4.69/36)×3, (4.82/37)×4” means that in one subframe, the cyclic prefix of 3 OFDM symbols is 4.69 us, corresponding to 36 sampling points; the cyclic prefix of 4 OFDM symbols is 4.82 us, corresponding to 37 sampling points. In the same way, the settings of cyclic prefixes with other bandwidths can be obtained.
The disadvantage of the above method is: from Table 1, it can be seen that each bandwidth corresponds to a setting of cyclic prefix, and the cyclic prefixes of different bandwidths are not compatible, which increase the complexity of system implementation but without any extra gain. Moreover, from Table 1, it also can be seen that 6 system bandwidths have 8 cyclic prefix lengths: 4.69, 5.21, 4.95, 4.82, 4.75, 4.73, 4.77 and 4.79 us, and since it is quite practical that the users with different bandwidths need to communicate with each other, each user should match with 8 different cyclic prefixes when accessing to the system, thus increasing the time to access to the system and the difficulty of implementation.
For another method, please refer to the OFDM system based on 802.16 standards. However, in the method mentioned in the system, the time lengths of the cyclic prefixes with different bandwidths are the same, and the distinctness that different data should use different cyclic prefixes is not considered. In OFDM communication system, there are two kinds of data sent to the users by the base station (BS): broadcast data and unicast data. The broadcast data means the data, such as the pilot frequency, important system messages, which should be received by all users in the service area of the BS. When the BS sends these messages, it should guarantee that the users in the bad channel environment (for example, those users at the edge of the cell) can correctly receive these important messages, and thus the BS should select OFDM symbols with relatively long cyclic prefix when sending these messages. The unicast message means the data sent to a certain user by the BS, and at this time, the BS should use OFDM symbol with relatively short cyclic prefix to ensure the system's efficiency.
Therefore, in the present OFDM system and standards, the compatibility of the time lengths of the cyclic prefixes with different bandwidths and the distinctness that different data should use different cyclic prefixes are not fully considered when setting the cyclic prefix, or only some aspect is considered while other aspects are neglected. Thus the prior art should be improved.